More particularly the invention relates to air barometers and variometers.
Due to the change in volume of the gas enclosed in the tank in dependence on temperature, the indication of gas pressure measuring apparatuses of the kind to which the invention relates is heavily dependent on temperature (about 2.6 mm mercury column per .degree.C change in temperature, if the pressure of the enclosed gas is 760 mm Hg at 20.degree.C). For this reason the field of application of gas pressure measuring apparatuses of this kind has hitherto been extremely limited, although they are cheap instruments of simple construction and high indicating sensitivity (it is easy to provide for an air barometer a scale division of 10 mm per mm of mercury for change in air pressure with only 10 cm.sup.3 enclosed gas volume).
Suggestings and attempts are known for automatically compensating the temperature dependence of the indication of gas pressure measuring apparatuses. They are based on the idea of compensating the differences in pressure, caused by changes in temperature, of the gas enclosed in the tank by the temperature-dependent level of a column of liquid which is either disposed as a pressure indicating element inside a capillary or tube system connected to the inside of the tank, or is layered above or below an indicating column of that kind and connected directly or indirectly via other columns of liquid to the gas in the tank. The level of the column of compensating liquid, in dependence on temperature, is made such, having regard to the coefficient of thermal expansion of the particular liquid, that the forces tending to produce displacement and caused only by a change in temperature cancel one another out.
The compensating devices on air barometers constructed on this principle take up a lot of space, are complicated and unstable in tilting, and more particularly the correct compensation depends on the correct position of the instrument and the compensating range is limited by the length of the columns of the liquid required for compensation. Moreover, this principle of compensation cannot be applied to variometers.